Investigators are currently examining in the laboratory the mechanisms which lead to sprouting of noradrenergic fibers in the spinal cord in models of chronic pain as well as the mechanisms that lead to a novel noradrenergic - cholinergic circuit in the spinal cord. In addition to examining the circumstances which generate this increased capacity for analgesia and the mechanisms by which they occur, investigators will test in this protocol whether approved and experimental treatments for neuropathic pain exploit this increased capacity.